Paper is sized in order to improve the resistance against water and other fluids. The two principal methods for sizing are internal sizing and surface sizing. Internal sizing comprises addition of suitable chemicals to the pulp whereby the chemicals either are absorbed on the cellulose fibres or react with the cellulose. By internal sizing a hydrophobic effect is obtained in the entire paper structure. The effect of surface sizing on the other hand is more or less restricted to the actual surface structure. The two methods are often used in combination.
The agents used for sizing are predominantly rosins, waxes, asphalt emulsions and a number of synthetic chemicals. The former agents are usually fixed to the cellulose fibres by precipitation with alum. The group synthetic sizing agents comprises e.g. alkyl ketene dimers which are chemically bound to the cellulose by reaction with the hydroxyl groups of the cellulose. Other synthetic sizing agents are anhydrides of carboxylic acids, such as stearic acid and alkyl succinic acid, isocyanates, carbamoyl chlorids etc.